Mazher Mahmood was found guilty of conspiring to pervert the course of justice in the case of pop star Tulisa Contostavlos.
Photograph: BBC

I have waited a long time to see Mazher Mahmood brought to book. Going back to 1988, when I was his boss at the Sunday Times, I discovered he had tried to pass the blame for his own error on to a news agency.

He went to extraordinary lengths to try to cover up his mistake and, when found out, quickly resigned in order to avoid being dismissed.

I have watched his career closely ever since. As recently as the Leveson inquiry, in 2012, he was still denying the truth of that Sunday Times incident.

The Old Bailey jury were not fooled either. And they are far from the first set of jurors to have seen through his lies and to be concerned about his questionable journalistic methods.

In 1999, a jury who were asked to convict a young peer, the Earl of Hardwicke, on a drugs charge took the extraordinary step of sending a note to the judge in which they said:

Had we been allowed to take the extreme provocation into account, we would undoubtedly have reached a different verdict.

Provocation was the name of Mahmood’s journalistic game. In almost every one of his investigative stories, the methodology was the same: induce the “target” to buy a quantity of drugs, publish the story “in the public interest” and turn the target into a “victim”.

Of course, those victims should not have broken the law. But what the Hardwicke jury realised, and I have written about in many other cases, is the disproportionate level of inducement involved.

Mahmood offered his targets just enough, and sometimes a great deal more than enough, to ensure that they would be coaxed into doing his bidding in order to win the prize, be it a job offer, money, fame or, in one case, a sexual liaison.

That bidding inevitably involved them obtaining a quantity of drugs. That they did so was foolish and, yes, a breach of the law. But in every case they were sorely tempted.

Worse by far was the way in which people were enticed into criminal activities by Mahmood’s use of agents provocateur. One, Florim Gashi, came forward to speak to the Guardian. Another person spoke to Panorama.

As Gashi said of Mahmood’s victims: “I tricked them.” By that time, with a record of dishonesty, it was easy for lawyers to dismiss his evidence. But his sincerity shone through and he has never deviated from his accounts of what he did on behalf of Mahmood.

Make no mistake: people suffered because of Mahmood’s stories. I have lost count of the solicitors who have contacted me down the years pleading for me to highlight the way in which their clients’ lives were wrecked by News of the World exposures.

I have given statements about Mahmood to more than half a dozen lawyers and then watched as the cases have come to nought, usually because judges refused to countenance historic evidence.

My piece in the Independent on Sunday on 16 April, 2006. Photograph: Clipshare

So the pattern of Mahmood’s methodology, which was the key to revealing his duplicity, did not emerge. I ploughed a lonely furrow for several years in what he rightly called my campaign against him. I was open about it. In 2006, the Independent on Sunday carried a piece by me headlined “Why I am out to nail Mazher Mahmood”.

If other journalists, most especially the police, had taken notice then, a lot of innocent people would have been spared from his brand of journalism.

As I have written before, and said often, he brought investigative journalism, and all tabloid journalism, into disrepute. Yet successive News of the World (and Sun on Sunday) editors stoutly defended him and his methods.

Did they not note the embarrassing courtroom reverses? The trial that followed the fake Victoria Beckham kidnap plot collapsed, as did the high-profile trial of three men prosecuted for buying a non-existent substance, “red mercury”, in order to build a non-existent “dirty bomb”.

But it was the low-profile cases, in which vulnerable people ended up in trouble, that break the heart. In 2005, an Albanian immigrant, Besnik Qema, was sentenced to four-and-half years in prison for obtaining cocaine and forged identity documents on behalf of a “woman” who contacted him through an Albanian-language internet chatroom.

The woman was Gashi, operating on Mahmood’s behalf. Qema spent years in jail before a lawyer decided to fight on his behalf and took his case to the Criminal Cases Review Commission.

After considering Qema’s story it declared the conviction unsafe and referred it back to the crown court in 2010. There, the prosecuting authorities offered no evidence and the conviction was quashed. But no action was taken against Mahmood. Why not?